In order to be able to put the handle wafer to a fixed electrical potential, an electrical contact is desired (an electrically conducting connection). This can be affected from the back side of the disc for example. However, the implementation requires an electrically conducting backside contact with which the disc fabrication as well as the mounting process (assembly) would become remarkably more complex.
Out of this reason, it is advantageous that an electrically conducting contact is realized starting from the front side.
In order to be able to utilize the advantage of the dielectric insulation of the SOI disc, among others for integrated smart power circuits in which different sections of the circuit are in part on remarkably different voltages (for example ground and 600V), also insulation structures are, however, required which electrically insulate the different sections of the circuit electrically from each other. Therein, a trench insulation is used frequently, in particular in technologies which use thick (for example 50 μm) SOI discs.
In the U.S. Pat. No. 5,314,841, a method is described in which a trench is etched through the active disc and through the buried oxide. The opened up surface in the handle disc is doped during the source/drain-implantation and is contacted by means of the normal metallization.
A similar structure is disclosed in the U.S. Pat. No. 5,945,712. The thin active silicon and the buried oxide (called insulation layer) are etched through and are contacted during the normal IC metallization.
A similar case is to be read in the U.S. Pat. No. 6,300,666. Also there, the thin active silicon and the buried oxide are etched through. The exposed area in the carrier wafer is doped by an implantation. The carrier wafer is electrically connected by means of the normal IC metallization by means of the normal silicidation/metallization,
These methods can only be applied to very shallow insulation trenches (“shallow trench insulation” in thin film SOI techniques). With ratios of depth to breadth of 10:1 and larger, no contacting of the carrier disc can be achieved in this way. In the known structures and methods, also the metallization within the trench is disadvantageous whereby no further high temperature processes above 400° C. are possible. It is, however, an essential point that a simultaneous production of the trench insulation and of a contact to the carrier disc is not possible with such structures and methods.
In the U.S. Pat. No. 6,794,716 B2, a method or a structure, respectively, is described wherein a trench which reaches until below the buried oxide, is filled with “metal” and tungsten and, in this way, a contact region in the handle wafer is conductively connected to a part of the active layer (“body” of the transistor), but, however, only a portion of the layer.
The following method is disclosed in the U.S. Pat. No. 6,649,964: Trenches having differing breadths are etched into a SOI disc. The trenches are, therein, deeper than the buried oxide and, so to speak, “penetrate through” those. Thereafter, a semiconductor layer, poly silicon or amorphous silicon are deposited and are doped by oblige implantation. By means of an anisotropic etching, a so-called spacer out of doped silicon is generated at the sidewall of the trench which extents from the upper surface of the SPO-disc into the handle wafer. Thereafter, a metallization is input and structured in some trenches, i.e. at least one further photo resist mask is required besides the structuring of the trenches. The trenches are filled by depositing silicone oxide, and the disc is planarized by means of a CMP process. This method has the following disadvantages. At least two structuring steps are required. Because of the completed metallization, no further high temperature processes can be carried out subsequently to this method. The handle disc is permanently connected to the active disc by the doped spacer, however, a contacting insulated from the active silicon disc of the handle disc is, thereby, not possible. In the described structure, all areas of the active layer are connected to the handle disc and, thereby, short circuited.
In the U.S. Pat. No. 6,521,947 structures or a method, respectively, is/are described wherein, at first, shallow insulation trenches (“shallow trench insulation”) are etched. As a result, silicone islands are produced which are isolated from each other and covered by nitride. Subsequently, an oxide layer is deposited. In areas to the side of silicone islands, a trench by means of a pure oxide etching is generated which trench extents through the deposited oxide layer and through buried oxide of the SOI disc. The trench which reaches through the oxide and ends at the substrate, is, thereafter, filled up by poly silicon. In this way, a structure has been produced in which insulation trenches as well as contacts to the handle disc are contained. However, this structure has the disadvantage that, for producing them, a method has to be used in which two different structuring steps and two different etching steps have to be used for the insulation trenches and the contact. On the one hand, this means effort and, on the other hand, this means that this structure cannot be used in this way for deep insulation trenches having a typical depth of 50 μm.
In the DE-A 10 20005 010 944, a method is described in which deep insulation trenches and deep contact trenches can be manufactured at the same time. Therein, trenches having differing breadth are etched at first. In a subsequent filling step, narrow trenches are completely filled up, broad trenches, however, only in part. The partially affected filling of the broad trenches serves, thereafter, as a mask for etching the buried oxide layer. By means of a filling with conductive material, for example by doped poly silicon, the handle disc can be electrically contacted from the upper surface of the disc. It is, however, disadvantage with this method that, as compared to the fabrication of insulation trenches, further process steps are required: etching of the buried layer and a second filling with conductive poly silicon.
The invention is based on the objective to utilize process steps available in the technology, for contacting the handle disc without additional effort in the method for fabricating the contacting.